In a wireless communication system, multiple antennae are used at both transmitting and receiving ends so as to obtain a higher rate by means of spatial multiplexing. Since the receiving end can obtain a channel matrix, through which transmitted signals pass, through channel estimation, even if each antenna transmits different data, the receiving end can still parse out the transmitted data on each antenna which has passed through Multiple-Input Multiple-Out-put (MIMO) channel matrix.
Compared with a method of directly parsing the transmitted data on each antenna using the channel matrix, a simple and efficient method is to use the pre-coding technology. When the transmitting end knows the channel status, the pre-coding technology can equivalently change the channel transfer matrix according to requirements. The transmitting end pre-codes data in layers, maps it to antennae and then sends it to the receiving end through a wireless channel. If accurate channel status information (CSI) can be known at the transmitting end, singular value decomposition can be implemented for specific channel matrices. Afterwards, a right singular matrix decomposed from the channel matrix is used as a pre-coding matrix, and the data in the layers is pre-coded. However, the CSI generally can only be acquired accurately and directly at the receiving end, and the transmitting end can only obtain the CSI through feedback of the CSI from the receiving end to the transmitting end. In a practical system, the cost will be huge if the CSI is fed back directly, and meanwhile it will be limited by the feedback overhead of the practical system. Therefore, a commonly used method is codebook-based feedback. That is, the receiving end selects a suitable code word from a codebook according to the current channel status and feeds an index value of the code word in the codebook set to the transmitting end, thereby greatly reducing the amount of feedback.
The codebook-based pre-coding is a form of channel qualification. Assuming that the limited feedback channel capacity is B bps/Hz, then the number of available code words is N=2B. The feature vector space of the channel matrix is qualified to constitute a codebook space ={F1, F2ΛFN}. The transmitting end and the receiving end collectively store or generate in real time this codebook  (which is the same for both the transmitting end and the receiving end). Taking the pre-coding technology used by the Long Term Evolution (LTE) system as an example, the receiving end selects an optimal code word {circumflex over (F)} from the codebook space  according to certain rules, and recommends an index i of the code word {circumflex over (F)} to the transmitting end. Herein, the index of the code word is referred to as the Precoding Matrix Indicator (PMI) in the codebook. The transmitting end can directly find the corresponding pre-coded code word {circumflex over (F)} for transmission according to this index i, or it may directly select the code word itself for transmission instead of accepting the code word recommended by the receiving end. The receiving end will be informed of the index of the actually used code word during the transmitting process.
The Long Term Evolution Advanced (LTE-A) system, which is the evolution standard of the LTE, supports a larger system bandwidth (up to 100 MHz), and is backward compatible with the existing standard of the LTE. In order to increase the coverage of cell edges and throughput, the LTE-A supports transmission up to 8 antennae and 8 layers in the downlink on the basis of LTE system, and proposes some technologies for feedback enhancement, which mainly improve the feedback accuracy of the codebook and can improve the utilization ratio of frequency spectrum of the International Mobile Telecommunications-Advance (IMT-Advance) system and alleviate the shortage of frequency spectrum resources. Currently a consensus reached in the conference of the 3GPP RAN1 group for the enhanced feedback technology will be described below.
1) A pre-coding/feedback structure of one sub-band consists of two matrices.
2) Each of the two matrices belongs to a separate codebook. The codebook is known in advance to both a base station and user equipment (UE). The codebook may vary in different times and different sub-bands.
3) One matrix represents the attribute of bandwidth or long-time channel, while the other matrix represents the attribute of the determined frequency band or short-time channel.
4) The codebook used is represented in a form of limited countable matrix sets, and each matrix is knowable to the UE and the base station.
Herein, it can be seen that a structure based on dual codebooks is proposed for feedback of channel information. Specifically, for one sub-band or multiple united sub-bands that needs the feedback of the channel information, the UE feeds back at least two pieces of PMI information, PMI1 and PMI2 respectively, to the base station, wherein PMI1 corresponds to a code word W1 in one codebook C1, while PMI2 corresponds to a code word W2 in another codebook C2. The base station has the same information of C1 and C2, and finds out the corresponding code words W1 and W2 from the corresponding codebooks C1 and C2 after receiving the PMI1 and PMI2, and obtains the channel information according to the appointed function rule F (W1, W2).
The size of the codebook generally determines the signaling overhead, and generally the larger the size of the codebook is, the better performance can be obtained. Therefore, the compromise between performance and overhead is generally taken into consideration in codebook design. In practical codebook design, many factors are generally considered, for example, the design is adapted to as many as possible channel environments and antenna configuration modes. Therefore, in a certain particular transmission condition, all code words in the codebook are not necessarily adapted to the current particular transmission condition, but the receiving end needs to traverse all the code words in the codebook when selecting a code word. This will not only bring increase of calculation complexity to the receiving end, but also increase occurrences of the case where the receiving end wrongly selects the code word due to some non-ideal processes, especially in the case of larger codebook design.